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Harvest Moon

September 27, 2004

Harvest MoonTonight and tomorrow night you can see the large Harvest Moon rising at sunset over the Bay Area. It will continue to rise shortly after sunset for the next few evenings.

The Harvest Moon got its name, appropriately enough, from farmers. Fall is the time when the crops that will sustain us through winter are harvested. This means long hours of work on the farm. Unfortunately, fall also means the hours of daylight are getting fewer as the Earth tilts away from the sun. The first full moon after the autumnal equinox (the point at which the number of daylight hours and nighttime hours are equal), which occurs every year in September, rises near sunset and gives farmers enough light to work a few extra hours harvesting their crop. Hence, it became known as the Harvest Moon.

When you look at the Harvest Moon rising in the east you may notice it looks a little different than when high in the sky on a typical night.

First, you may notice it looks orange or red. When the moon is low on the horizon, light reflecting off the moon has to travel through much more of the Earth's atmosphere to reach your eye than it does when overhead. Our atmosphere contains dust and water and other tiny objects that scatter light. Our atmosphere scatters short wavelength light (blues, violets, greens) more effectively than long wavelength light (reds, oranges, yellows) so the more atmosphere the light has to travel through the redder it looks to us.

Perhaps the biggest difference you'll notice is how much larger it appears. This is actually an illusion. There are several theories that describe why the moon looks so much larger when at the horizon than it does when high in the nighttime sky. While many will ascribe a very basic reasoning, it is actually a quite complex combination of factors that contribute to this illusion. If you want the whole truth with a detailed explanation then please visit, Professor Don McCready's The Moon Illusion Explained website. 

Tomorrow night, Tuesday, September 28th, the moon is at its fullest and, weather permitting, should be at its most spectacular. So go outside at sunset and see for yourself. Remember to take along your camera. The harvest moon makes the perfect backdrop for your fall photos.


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